In recent years, systems which generate electric power by converting natural energy such as wind power or sunlight into electric energy are increasingly introduced, for the purpose of reducing environmental loads. Such power dependent on natural energy is greatly influenced by weather conditions. Therefore, power generated from natural energy easily fluctuates. If a system cannot sufficiently adjust to fluctuations in the generated power, a voltage fluctuation or power cut occurs due to the load.
If a thermal power plant is put on standby as a substitute power to avoid such a situation, however, a maintenance cost such as a fuel cost increases. Therefore, a power generation system using natural energy demanded to be capable of restricting power fluctuations by itself.
On the other side, hybrid cars and electric cars are rapidly being introduced in connection with promotion of environmental load reduction. Batteries are mounted as power sources in these cars. Therefore, in the future, a large number of secondary batteries are expected to be disposed of when the cars are discarded. Consequently, dealing with those secondary batteries is also expected to cause problems.
However, a problem described below occurs if a power-fluctuation reducing apparatus which reduces power fluctuations in a power generation system by charging/discharging secondary batteries is configured by using secondary batteries to be reused, in view of the situation described above.
There are various types of secondary batteries. If batteries of an identical type are made of different terminal materials or by different manufacturers, the batteries have respectively different voltages between ends of the batteries. Further, individual differences increase between reused secondary batteries due to use conditions, use periods, use environments, or the like.
When secondary batteries of such different styles are used to construct a power-fluctuation reducing apparatus, partial heat generation may be caused by a circulation current between batteries or a stored power amount may be decreased due to unevenness in voltage allotment between constitutive batteries.